Would you rather have $50 now or $100 two weeks from now? Even though the $100 is obviously the better choice, many people will opt for the $50. Both humans and animals show this tendency to place lower value on later rewards, a behavior known as temporal discounting. High rates of temporal discounting can lead to impulsive behavior, and at its worst, too much of this “now bias” is associated with pathological gambling, attention deficit hyperactivity disorder and drug addiction.
What determines if you’ll be an impulsive decision-maker? New evidence suggests that for women, estrogen levels might be a factor. In a recent study published in the Journal of Neuroscience, Charlotte Boettiger and her team at the University of North Carolina revealed that greater increases in estrogen levels across the menstrual cycle led to less impulsive decision making.
The researchers tested the “now bias” in 87 women between the ages of 18 and 40 at two different points in their menstrual cycle – in the menstrual phase when estrogen levels are low and the follicular phase when estrogen levels are high. Participants were given a delay-discounting task where they had to choose between two options: a certain sum of money at a later date or a discounted amount immediately (e.g. $100 in one week or $70 today). Subjects showed a greater bias toward the immediate choice during the menstrual phase of the cycle, when estrogen levels were low.
Estrogen levels vary between women and can change with factors like stress and age. When the researchers measured amounts of estradiol (the dominant form of estrogen) from the saliva in a subset of the participants at the two points in their menstrual cycles, they found that not all of them showed a detectable increase. Only those with a measureable rise in estradiol showed a significant change in impulsive decision-making.
The overall level of an individual’s impulsivity is also affected by estrogen. In this study, those with lower estrogen levels in the first session showed higher measures of trait impulsivity as measured by the Barratt Impulsiveness scale (you can measure your level of impulsivity here).
The cycle of fertility
Women’s estrogen levels rise and fall during their monthly ovarian cycles. The cycle begins with menstruation, where estrogen levels are low. As ovulation draws near, the woman enters the follicular phase and estrogen levels progressively increase until they peak near mid-cycle (around 11-12 days) when fertility is at its highest.
If impulsive choice decreases with increasing estrogen levels, women may be more careful about the decisions they make during peak fertility periods. This could potentially have an effect on decisions ranging from gambling to mate selection.
It’s possible that paying more attention to long-term consequences when reproduction is most likely to happen holds an evolutionary advantage. But this cyclic effect may also be a mere coincidence. “Evolution is not an optimizer,” says Boettiger, “and if nature figures out a system that works well enough, that’s what we adjust to. There may not be an evolutionary purpose to biological function, even though it’s tempting to speculate that there is.”
A role for dopamine
Dopamine is one of the key regulators of decisions, and impaired decision-making is seen in people with dopamine-related disorders such as addiction, schizophrenia and Parkinson’s disease.
Previous animal studies have found evidence for a link between estrogen and dopamine signalling. Researchers discovered that ovariectomies changed dopamine concentration in rats, and estrogen increased dopamine innervation in the primate prefrontal cortex.
Though scientists have yet to find direct evidence for the interaction between dopamine and estrogen in the human brain, various imaging and behavioral studies have begun to shed light on this relationship. For instance, a 2001 study found sex differences in dopamine receptor levels in the brain, which could be attributed to differences in levels of sex steroids (e.g. estrogen and testosterone). To date, most studies looking at sex steroids have been focused on men, who are easier to study because their hormone levels don’t normally fluctuate as much as women’s do, but research on women is beginning to slowly emerge.
It’s in your genes
Genetics can determine how much dopamine you have in your brain. Catechol-O-methyltransferase (COMT) is an enzyme that metabolizes dopamine in the prefrontal cortex, clearing it from synapses and decreasing the amount available for use. A common polymorphism (Val158Met) in the COMT gene replaces Valine (Val) with Methionine (Met). Val carriers have a four-fold increase in dopamine metabolism, which leads to lower baseline levels of dopamine in the cortex than those without Val.
Boettiger and her team assessed COMT genotypes in the tested individuals and found that Val carrier status predicted decreased impulsivity when an increase in estrogen was found, suggesting the interaction between dopamine and estrogen influences behavior.
A 2011 study at The University of California, Berkeley found similar estrogen-mediated effects on working memory, another dopamine dependent cognitive function, in Val carriers. Estrogen levels influenced performance on working memory, and this depended on the individual’s baseline dopamine level.
Drugs and decisions
According to Boettiger, the most critical implications of this study may be for women who are naturally cycling while also on medications that modulate dopamine signalling or adjust hormone levels. Dopaminergic medications such as antidepressants and ADHD medications are clinically prevalent, and birth control pills work by artificially enhancing levels of estrogen throughout the menstrual cycle. Further studies may look at how these drugs or artificially increasing levels of estrogen affect decision making. “It’s something that’s been investigated very little, but worth looking into,” says Boettiger.
If you’re a woman, the ability turn down immediate rewards might decline as the “time of the month” draws near. Whether it holds an evolutionary advantage or is just a random act of nature, the amount of estrogen in your system could affect whether you’ll be able to forgo immediate pleasure for later rewards.
Smith, C. T., Sierra, Y., Oppler, S. H., & Boettiger, C. A. (2014). Ovarian Cycle Effects on Immediate Reward Selection Bias in Humans: A Role for Estradiol.The Journal of Neuroscience, 34(16), 5468-5476.
Xiao, L., & Becker, J. B. (1994). Quantitative microdialysis determination of extracellular striatal dopamine concentration in male and female rats: effects of estrous cycle and gonadectomy. Neuroscience letters, 180(2), 155-158.
Kritzer, M. F., & Kohama, S. G. (1999). Ovarian hormones differentially influence immunoreactivity for dopamine ?-hydroxylase, choline acetyltransferase, and serotonin in the dorsolateral prefrontal cortex of adult rhesus monkeys. Journal of comparative neurology, 409(3), 438-451.
Kaasinen, V., N?gren, K., Hietala, J., Farde, L., & Rinne, J. O. (2001). Sex differences in extrastriatal dopamine D2-like receptors in the human brain.American Journal of Psychiatry, 158(2), 308-311.
Jacobs, E., & D’Esposito, M. (2011). Estrogen shapes dopamine-dependent cognitive processes: implications for women’s health. The Journal of Neuroscience, 31(14), 5286-5293.